1. Field of the Invention
The present invention generally relates to monolithic radio frequency (RF) microwave integrated circuits, and more specifically to a multiple device structure used for RF power amplifications using a heterojunction bipolar transistor (HBT).
2. Description of the Related Art
Heterojunction bipolar transistors enable more efficient RF power amplification than other semiconductor devices in integrated circuit form. Extremely high power added efficiency can be obtained because of the high power density and high breakdown of the HBT devices. For high power designs, a multitude of devices are used in some form of parallel structure in order to distribute the power over a sufficient area such that excessive heating is not present to degrade the performance or reliability of the devices.
During normal operation, the current is equally distributed through the many HBTs and excessive heat and other problems do not result. However, if the HBTs are even slightly mismatched, one HBT will operate at a higher temperature than the others and draw a larger amount of current. Since the combined current of all devices is much more than enough to cause destruction of the hot HBT, the possibility exists for what is called in the art, "thermal runaway." Thermal runaway results when one device fails, causing a chain reaction failure of other components. Unfortunately, small differences in devices or placement can cause an imbalance of heating between the individual devices. Any bipolar device which is connected in parallel with other similar devices and which is hotter than its neighbors will tend to draw more current, thus heating itself even more. The heating compounds itself and the result is a thermal runaway phenomenon which will destroy the device and the integrated circuit (IC) itself.
Two prior art circuit techniques which attempt to avoid this problem are the use of an emitter ballast resistor (FIG. 2), and the use of cascode device cells (FIG. 3). In FIG. 2, the ballast resistors degenerate the gain of the device such that increased collector current tends to increase emitter voltage and thereby decrease the emitter-base bias voltage, hence reducing current. A thermally stable circuit can be achieved by making the emitter ballast resistors sufficiently large to prevent this degeneration.
Another method is to use cascode devices as shown in FIG. 3. In this circuit, most of the voltage is impressed on the upper (common base) transistor in each pair. Therefore, most of the heat is generated in this device instead of the common emitter amplifier, which can now operate at a much lower temperature and thereby minimizing the possibility of thermal runaway.
The prior art designs, however, contain limits on efficiency. In the above circuits, other devices are used to achieve thermal stability which are placed in series with the output device. It is desired that as much power taken from the DC power supply as possible is transformed into RF power at the output of the circuit. Any power consumed in the circuit itself, therefore, is wasted and results in degraded overall efficiency. In FIG. 2, the DC and AC collector current that drives the output must pass through the emitter ballast resistors. Therefore, significant power is dissipated in those resistors. This translates to wasted power, and hence reduced efficiency of the amplifier. Since gallium arsenide generally is not a good heat conductor, the emitter ballast resistors have to be quite large to thermally stabilize the circuit and the efficiency loss is commensurably large.
In a cascode circuit (FIG. 3), another transistor is placed in series with the output. The effect is the same as above, i.e., the extra transistor consumes power hence reducing efficiency. The efficiency loss is quite severe because the common emitter's collector must be maintained at a fairly high voltage (one volt or more) in order for that transistor to stay out of saturation that would reduce gain and linearity. Therefore, the cascode arrangement is inferior to the emitter ballast method from a thermal stability point of view.
Thus there is a need in the art for an efficient and linear amplifier which can be built in HBT integrated circuit technology.